Handling solid waste is an increasingly difficult problem. A majority of the United States' solid waste is disposed of in landfills. However, nationwide the availability of solid waste landfills is decreasing as landfills reach capacity and close, while opening new landfills is difficult due to regulations. The national average for solid waste disposal fees has increased several times since 1985. Such fees, called tipping fees, are expected to rise even further with increased demand for land fill space.
These and other issues pertaining to the environmental impact of landfills have prompted solid waste managers to seek methods to reduce volumes and disposal costs of municipal solid waste (MSW). Some experts contend that there is no waste volume problem, but there exists a sorting and recycling problem.
There have been a number of suggestions, some patented, for treating and/or reprocessing MSW. These often involve some form of heat and pressure, and often a reaction vessel that may rotate. Although MSW varies in composition, there is a certain norm to its content. The cellulosic fraction represents approximately seventy percent of the bulk of typical MSW. Other forms of waste have a cellulosic fraction and can be processed in the same process that works for MSW or other biomass bearing material. These include sewer sludge, industrial waste streams, industrial byproducts, agricultural waste and byproducts, food processing waste and byproducts, and other sources. The cellulosic fraction of the biomass bearing material includes all portions that are organic, putrescible, or of biological origin which could eventually decay in a landfill and the decay could eventually lead to the production of methane gas and leachate. It is to be understood that the use of the term “biomass bearing material” in this application encompasses MSW as well as other forms of waste having a cellulosic fraction.
Separating this cellulosic fraction into reusable products can alleviate much of the pollution resulting from the disposal of organic bearing waste. The quality of the cellulosic fraction that is separated from the biomass bearing material determines how it may be used. Higher quality cellulosic fractions bearing less contamination from plastic and other materials will have more uses and be less regulated than lower quality fractions. For example, in the energy industry, solid waste may be mass burned as a fuel source in specially built incinerators. The energy industry may also utilize refuse derived fuels which have had some separation of metal and glass materials before burning. Due to the remaining plastic contaminants in solid waste and refuse derived fuels, these incinerators are also highly regulated and expensive to maintain. However, high quality cellulosic material that has very low contamination from plastic materials may be certified as a renewable organic based fuel that is not as highly regulated. The high quality cellulosic fraction of biomass bearing material may be used as loose or pelletized fuel or feedstock for gasification, bio refineries and conversion into ethanol and other chemicals and fuels. The high quality biomass bearing fraction of MSW could also serve as a source of hydrogen and other liquid, elemental or gaseous products achieved through gasification or other thermal processes, including hydrolysis. In summary, the more effectively that the cellulosic fraction can be removed from the biomass bearing material, the less the waste is likely to contaminate the environment and the more likely the removed fraction can be better utilized on other processes.
Once the cellulosic fraction is removed, the remaining waste material, which includes ferrous and non-ferrous metals, plastics, and textiles, is much reduced in volume and other recoverable fractions of the waste stream can be recovered. Processes are available to recover each of these materials and send them to their respective market. If the cellulosic and recoverable waste streams are removed from biomass bearing material, the volume of such waste streams going to landfills is typically less than fifteen percent of the original amount. This reduction in volume reduces hauling costs and landfill space requirements. Removal of the cellulosic fraction of the biomass bearing material also accomplishes this reduction while reducing polluting effluents, including methane and leachate. Further, if the cellulosic portion is removed from the biomass bearing material, the remaining fraction of inorganic waste could be disposed of in waste sites reserved for construction or other inorganic waste, which is space that is often less regulated and therefore less expensive.
Rotating reaction vessels for older forms of solid waste processing techniques are generally very large steel structures that rotate along their longitudinal access. These reaction vessels have eccentricities in the axis of rotation that are introduced during the manufacture and installation of the vessel or are caused by eccentric or other expansion of the metal vessel during operation. The eccentricities can result in the centerline of the vessel moving a significant distance during rotation of the vessel. These eccentricities can be manifest as asymmetric changes in the vessel body relative to the door opening. These asymmetric changes can decrease the operating efficiency of the vessel by increasing the difficulty of forming an effective seal between the vessel and the door. Moreover, the eccentric movement of the vessel can increase operating costs of the vessel due to stresses that can be transferred to door supporting structures during operation. Thus, a reaction vessel is needed that has decreased eccentricities and/or includes structures that accommodate the eccentricities so as to assist in forming the seal between the door and the vessel and to decrease the stresses transferred to the door supporting structures during operation.
Many prior reaction vessels for treating biomass bearing material utilize inefficient high-pressure steam systems to cook and soften the cellulosic fibers of the biomass bearing material. High-pressure steam vessels are undesirable because 1) they require significant licensing and inspection regimes; 2) they must be built to withstand higher pressure which greatly increases their cost; 3) they require a great amount of energy to generate and maintain the high operating pressures, which results in high operating costs; 4) cycling between the high operating pressures and the emptying pressure results in the inefficient loss of a large amount of energy; 5) high operating pressures present significant safety concerns; and 6) high pressure, high pressure steam results in contamination of the cellulosic fraction such as by melted plastic materials that decreases the quality of the fraction and limits its use as a feedstock for various processes. For example, a cellulosic fraction contaminated with plastic material (or chemicals that are leached from plastic materials when exposed to high pressure and high temperature steam) decrease the likelihood that the fraction can certified as a renewable source of energy under various regulatory guidelines as described above or that it could be used as a feedstock in the chemical or paper reprocessing industries.
In today's industrial world, time, energy, and safety are vital factors when choosing to employ a new industrial system. New industrial systems must operate efficiently and safely and also minimize interaction with their operators so that it can function on a day to day basis in the real world. A reaction vessel that achieves adequate softening, pulverization, and separation of cellulosic fibers while utilizing low pressure steam meet these objectives because the vessel could be lighter, less expensive to construct and operate, have decreased cycling times, and be more efficient and safer. An improved lower pressure reaction vessel that achieves adequate softening, pulverization, and separation of cellulosic fibers with greater efficiency and safety is thus needed.
More specifically, what is needed is an improved reaction vessel that agitates the biomass bearing material to quickly saturate the biomass bearing material with low pressure steam thereby allowing the separation of and collection of a high quality cellulosic fraction from the other fractions of the waste. The vessel should be safe to operate and efficiently use the heat and steam so that the moisture content of the product is low, and provides a sterilization effect upon the biomass bearing material, imparts a chemical change to the biomass bearing material and imparts beneficial handling, flow, and chemical characteristics to the product of processing in the reaction vessel, which thus converts the biomass from waste into a feedstock for subsequent processes. Additionally, the non biomass fractions can benefit by removing labels from containers, fracturing glass for possible later separation from the waste stream, compacting and agglomerating plastics, and compacting ferrous and non-ferrous materials.